Motivation for this change

Nix works on MacOS, Dart works on MacOS, so let's make Dart available through Nix on MacOS too. Later, we can do the same for Flutter.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS (n/a: only adding support for Darwin.)
  • macOS

    $ cat /etc/nix/nix.conf
    build-users-group = nixbld
    sandbox = true
    $ for v in 1.24.3 2.7.2 2.10.0 2.9.0-4.0.dev 2.11.0-161.0.dev ; do
      nix-build -E "with import <nixpkgs> {}; callPackage ./default.nix {version=\"$v\";}"
      r="$r $v:$?"
    done ; echo "$r"
    /nix/store/mw3v7784iz85gp2rw8j1gld6qsjnwcji-dart-1.24.3
    /nix/store/pnzn2vdpp7m0xxmwlwnghq57fh2fkn6b-dart-2.7.2
    /nix/store/83pvpas75ancdfxrghw6lzkkzai3gim9-dart-2.10.0
    /nix/store/s0k4p9b8pxw0fd0pp4h0ziz1xx44dmz1-dart-2.9.0-4.0.dev
    /nix/store/76znscxn89433yc9n80hwh7p472czaky-dart-2.11.0-161.0.dev
     1.24.3:0 2.7.2:0 2.10.0:0 2.9.0-4.0.dev:0 2.11.0-161.0.dev:0
    

  • other Linux distributions (n/a: only adding support for Darwin.)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests) (n/a: there are no relevant tests yet.)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"

  [...]
  No diff detected, stopping review...
  [...]
  

• Tested execution of all binary files (usually in ./result/bin/)

  $ for v in 1.24.3 2.7.2 2.10.0 2.9.0-4.0.dev 2.11.0-161.0.dev ; do
      nix-build -E "with import <nixpkgs> {}; callPackage ./default.nix {version=\"$v\";}" && for f in result/bin/* ; do
        if [ -f $f ]; then
          $f --help 2>/dev/null 1>/dev/null || ( echo "\$ $f --help"; $f --help )
        fi
      done 
    done
  /nix/store/mw3v7784iz85gp2rw8j1gld6qsjnwcji-dart-1.24.3
  /nix/store/pnzn2vdpp7m0xxmwlwnghq57fh2fkn6b-dart-2.7.2
  /nix/store/83pvpas75ancdfxrghw6lzkkzai3gim9-dart-2.10.0
  $ result/bin/dartdevc --help
  Please specify the output file location. For example:
      -o PATH/TO/OUTPUT_FILE.js
  /nix/store/s0k4p9b8pxw0fd0pp4h0ziz1xx44dmz1-dart-2.9.0-4.0.dev
  $ result/bin/dartdevc --help
  Please specify the output file location. For example:
      -o PATH/TO/OUTPUT_FILE.js
  /nix/store/76znscxn89433yc9n80hwh7p472czaky-dart-2.11.0-161.0.dev
  $ result/bin/dartdevc --help
  Please specify the output file location. For example:
      -o PATH/TO/OUTPUT_FILE.js
  

• Determined the impact on package closure size (by running nix path-info -S before and after)
  • (I'm not sure how to do that just yet!)
• Ensured that relevant documentation is up to date (n/a: I searched and haven't found any relevant documentation.)
• Fits CONTRIBUTING.md.